Conventionally, as shown in FIG. 5, a paint drying furnace as noted above has, acting as furnace interior heating means Hb, Hc for heating, to a high temperature, gases RA' returned from furnace interior circulating gas passages 9b, 9c to furnace interiors 1b, 1c, combustion type heating devices 19b', 19c' disposed on the furnace interior circulating gas passages 9b, 9c for heating gases RA circulating through the furnace interior circulating gas passages 9b, 9c by burning operation of burners b.
As the above combustion type heating devices 19b', 19c' for the furnace interiors, it is necessary to employ indirect heating type, combustion type heating devices in which burning flames and combustion gas G produced by the burning operation of burners b and the gases RA circulating through the furnace interior circulating gas passages 9b, 9c to be heated exchange heat in a non-contact mode through inner heat exchangers hx.
Where direct heating type, combustion type heating devices are employed as the combustion type heating devices 19b', 19c' for the furnace interiors, which burn a fuel directly in the atmosphere of gases RA circulating through the furnace interior circulating gas passages 9b, 9c, paint solvent vapor generated in the furnace interiors 1b, 1c during a baking and drying process and included in the gases RA circulating through the furnace interior circulating gas passages 9b, 9c is directly exposed and reacts to the burning flames in the combustion type heating devices 19b', 19c', to produce a reaction product which lowers paint film quality (i.e. a reaction product which adheres to the paint films after return to the furnace interiors 1b, 1c to lower paint film quality). The above construction is employed in order to prevent formation of such a reaction product. In FIG. 5, 8b, 8c denote furnace interior exhaust passages for discharging as exhaust gas EA from the system, part of furnace interior gases ZA withdrawn from the furnace interiors 1b, 1c. 18b', 18c' denote fresh air passages for mixing fresh air OA (usually ambient air) in a quantity corresponding to the exhaust gas from the furnace interior exhaust gas passages 8b, 8c into the gases RA circulating through the furnace interior circulating gas passages 9b, 9c to dilute the solvent vapor produced in the furnace interiors 1b, 1c.
However, the above conventional furnace discharges from the system the combustion gas G retaining a large amount of heat after the heat exchange in the indirect heating type, combustion type heating devices 19b', 19c' with the gases RA circulating through the furnace interior circulating gas passages 9b, 9c (specifically, the circulating gases mixed with fresh air OA), and thus involves a great heat loss. Moreover, the indirect heating type, combustion type heating devices 19b', 19c' including the inner heat exchangers hx have a large heat capacity, and require a large heating load in start-up times. These points pose a problem of high running cost.
In addition, the indirect heating type, combustion type heating devices 19b', 19c', with the inner heat exchangers hx, have a large, complicated construction, which poses a problem of requiring high apparatus cost and large installation space.
Having regard to the state of the prior art noted above, a primary object of this invention is to reduce the heat loss noted above while preventing formation of a reaction product which lowers paint film quality.
Another object is to reduce the heating load in start-up times, and yet to downsize and simplify the apparatus construction.